Formation of melt pocket in mantle peridotite xenolith from western Qinling, Central China: Partial melting and metasomatism

Abstract

Two types of melt pockets, closed melt pocket (CMP) and open melt pocket (OMP), are recognized from the peridotite xenoliths entrained in the Cenozoic kamafugites in western Qinling (秦岭), Central China. The Haoti (好梯) CMPs have a mineral assemblage of olivine+ clinopyroxene+amphibole+K-feldspar, whereas the Baiguan (白关) CMPs are composed of olivine+clinopyroxene+ilmenite+carbonate. The components of the OMPs are more complicated. In the Haoti OMPs, there are olivine, clinopyroxene, glass, low modal abundances of amphibole, K-feldspar (Kfs), ilmenite, sulfide, chlorite, perovskite, chromite and phlogopite. The Baiguan OMPs contain olivine, clinopyroxene, glass, chlorite and chromite. Compositionally, olivines in the CMPs and OMPs are both apparently depleted in Ni, and those in the OMPs are also depleted in Fe and Mg, and enriched in Ca compared to the primary ones. Clinopyroxenes display large and systematical compositional variations between the CMPs and OMPs, particularly in Al, Cr, Na, Ca and Ti. Glasses are generally depleted in Si compared to the worldwide glasses in melt pockets, although they still have large variations. Amphiboles and K-feldspars have relatively restricted compositional variations. The petrographical observations and mineral chemistry suggest that the Haoti and Baiguan CMPs were generated by the in-situ decompression melting of orthopyroxenes, olivines and clinopyroxenes, and by the addition of minor external K-rich and Ca-rich melt/fluids. The OMPs formed during the latest metasomatic event in the lithospheric mantle beneath the western Qinling.